from __future__ import annotations

import codecs
import math
from collections.abc import AsyncIterator
from typing import Any, Callable, Literal, overload

import trio

from ..exceptions import ConcurrencyError
from ..frames import OP_BINARY, OP_CONT, OP_TEXT, Frame
from ..typing import Data


__all__ = ["Assembler"]

UTF8Decoder = codecs.getincrementaldecoder("utf-8")


class Assembler:
    """
    Assemble messages from frames.

    :class:`Assembler` expects only data frames. The stream of frames must
    respect the protocol; if it doesn't, the behavior is undefined.

    Args:
        pause: Called when the buffer of frames goes above the high water mark;
            should pause reading from the network.
        resume: Called when the buffer of frames goes below the low water mark;
            should resume reading from the network.

    """

    def __init__(
        self,
        high: int | None = None,
        low: int | None = None,
        pause: Callable[[], Any] = lambda: None,
        resume: Callable[[], Any] = lambda: None,
    ) -> None:
        # Queue of incoming frames.
        self.send_frames: trio.MemorySendChannel[Frame]
        self.recv_frames: trio.MemoryReceiveChannel[Frame]
        self.send_frames, self.recv_frames = trio.open_memory_channel(math.inf)

        # We cannot put a hard limit on the size of the queue because a single
        # call to Protocol.data_received() could produce thousands of frames,
        # which must be buffered. Instead, we pause reading when the buffer goes
        # above the high limit and we resume when it goes under the low limit.
        if high is not None and low is None:
            low = high // 4
        if high is None and low is not None:
            high = low * 4
        if high is not None and low is not None:
            if low < 0:
                raise ValueError("low must be positive or equal to zero")
            if high < low:
                raise ValueError("high must be greater than or equal to low")
        self.high, self.low = high, low
        self.pause = pause
        self.resume = resume
        self.paused = False

        # This flag prevents concurrent calls to get() by user code.
        self.get_in_progress = False

        # This flag marks the end of the connection.
        self.closed = False

    @overload
    async def get(self, decode: Literal[True]) -> str: ...

    @overload
    async def get(self, decode: Literal[False]) -> bytes: ...

    @overload
    async def get(self, decode: bool | None = None) -> Data: ...

    async def get(self, decode: bool | None = None) -> Data:
        """
        Read the next message.

        :meth:`get` returns a single :class:`str` or :class:`bytes`.

        If the message is fragmented, :meth:`get` waits until the last frame is
        received, then it reassembles the message and returns it. To receive
        messages frame by frame, use :meth:`get_iter` instead.

        Args:
            decode: :obj:`False` disables UTF-8 decoding of text frames and
                returns :class:`bytes`. :obj:`True` forces UTF-8 decoding of
                binary frames and returns :class:`str`.

        Raises:
            EOFError: If the stream of frames has ended.
            UnicodeDecodeError: If a text frame contains invalid UTF-8.
            ConcurrencyError: If two coroutines run :meth:`get` or
                :meth:`get_iter` concurrently.

        """
        if self.get_in_progress:
            raise ConcurrencyError("get() or get_iter() is already running")
        self.get_in_progress = True

        # Locking with get_in_progress prevents concurrent execution
        # until get() fetches a complete message or is canceled.

        try:
            # Fetch the first frame.
            try:
                frame = await self.recv_frames.receive()
            except trio.EndOfChannel:
                raise EOFError("stream of frames ended")
            self.maybe_resume()
            assert frame.opcode is OP_TEXT or frame.opcode is OP_BINARY
            if decode is None:
                decode = frame.opcode is OP_TEXT
            frames = [frame]

            # Fetch subsequent frames for fragmented messages.
            while not frame.fin:
                try:
                    frame = await self.recv_frames.receive()
                except trio.Cancelled:
                    # Put frames already received back into the queue
                    # so that future calls to get() can return them.
                    # Bypass the statistics() method for performance.
                    state = self.send_frames._state
                    assert not state.receive_tasks, "no task should receive"
                    assert not state.data, "queue should be empty"
                    for frame in frames:
                        self.send_frames.send_nowait(frame)
                    raise
                except trio.EndOfChannel:
                    raise EOFError("stream of frames ended")
                self.maybe_resume()
                assert frame.opcode is OP_CONT
                frames.append(frame)

        finally:
            self.get_in_progress = False

        # This converts frame.data to bytes when it's a bytearray.
        data = b"".join(frame.data for frame in frames)
        if decode:
            return data.decode()
        else:
            return data

    @overload
    def get_iter(self, decode: Literal[True]) -> AsyncIterator[str]: ...

    @overload
    def get_iter(self, decode: Literal[False]) -> AsyncIterator[bytes]: ...

    @overload
    def get_iter(self, decode: bool | None = None) -> AsyncIterator[Data]: ...

    async def get_iter(self, decode: bool | None = None) -> AsyncIterator[Data]:
        """
        Stream the next message.

        Iterating the return value of :meth:`get_iter` asynchronously yields a
        :class:`str` or :class:`bytes` for each frame in the message.

        The iterator must be fully consumed before calling :meth:`get_iter` or
        :meth:`get` again. Else, :exc:`ConcurrencyError` is raised.

        This method only makes sense for fragmented messages. If messages aren't
        fragmented, use :meth:`get` instead.

        Args:
            decode: :obj:`False` disables UTF-8 decoding of text frames and
                returns :class:`bytes`. :obj:`True` forces UTF-8 decoding of
                binary frames and returns :class:`str`.

        Raises:
            EOFError: If the stream of frames has ended.
            UnicodeDecodeError: If a text frame contains invalid UTF-8.
            ConcurrencyError: If two coroutines run :meth:`get` or
                :meth:`get_iter` concurrently.

        """
        if self.get_in_progress:
            raise ConcurrencyError("get() or get_iter() is already running")
        self.get_in_progress = True

        # Locking with get_in_progress prevents concurrent execution
        # until get_iter() fetches a complete message or is canceled.

        # If get_iter() raises an exception e.g. in decoder.decode(),
        # get_in_progress remains set and the connection becomes unusable.

        # Yield the first frame.
        try:
            frame = await self.recv_frames.receive()
        except trio.Cancelled:
            self.get_in_progress = False
            raise
        except trio.EndOfChannel:
            raise EOFError("stream of frames ended")
        self.maybe_resume()
        assert frame.opcode is OP_TEXT or frame.opcode is OP_BINARY
        if decode is None:
            decode = frame.opcode is OP_TEXT
        if decode:
            decoder = UTF8Decoder()
            yield decoder.decode(frame.data, frame.fin)
        else:
            # Convert to bytes when frame.data is a bytearray.
            yield bytes(frame.data)

        # Yield subsequent frames for fragmented messages.
        while not frame.fin:
            # We cannot handle trio.Cancelled because we don't buffer
            # previous fragments — we're streaming them. Canceling get_iter()
            # here will leave the assembler in a stuck state. Future calls to
            # get() or get_iter() will raise ConcurrencyError.
            try:
                frame = await self.recv_frames.receive()
            except trio.EndOfChannel:
                raise EOFError("stream of frames ended")
            self.maybe_resume()
            assert frame.opcode is OP_CONT
            if decode:
                yield decoder.decode(frame.data, frame.fin)
            else:
                # Convert to bytes when frame.data is a bytearray.
                yield bytes(frame.data)

        self.get_in_progress = False

    def put(self, frame: Frame) -> None:
        """
        Add ``frame`` to the next message.

        Raises:
            EOFError: If the stream of frames has ended.

        """
        if self.closed:
            raise EOFError("stream of frames ended")

        self.send_frames.send_nowait(frame)
        self.maybe_pause()

    def maybe_pause(self) -> None:
        """Pause the writer if queue is above the high water mark."""
        # Skip if flow control is disabled.
        if self.high is None:
            return

        # Bypass the statistics() method for performance.
        # Check for "> high" to support high = 0.
        if len(self.send_frames._state.data) > self.high and not self.paused:
            self.paused = True
            self.pause()

    def maybe_resume(self) -> None:
        """Resume the writer if queue is below the low water mark."""
        # Skip if flow control is disabled.
        if self.low is None:
            return

        # Bypass the statistics() method for performance.
        # Check for "<= low" to support low = 0.
        if len(self.send_frames._state.data) <= self.low and self.paused:
            self.paused = False
            self.resume()

    def close(self) -> None:
        """
        End the stream of frames.

        Calling :meth:`close` concurrently with :meth:`get`, :meth:`get_iter`,
        or :meth:`put` is safe. They will raise :exc:`EOFError`.

        """
        if self.closed:
            return

        self.closed = True

        # Unblock get() or get_iter().
        self.send_frames.close()
